Nitrogen is colorless, odorless and tasteless, but all life on earth
depends on it. Without it, our bodies cannot synthesize the nucleic acids that
make up our DNA, or the protein-forming amino acids that are the very building
blocks of our cells. Problematically, atmospheric nitrogen is relatively inert
or nonreactive. This has created a unique biological dependency on a process
called nitrogen fixation—where inert nitrogen from the atmosphere is converted
into more reactive ammonia, a major component of soil fertilizers. A recent
discovery has revealed just how reliant recovering forests are on nitrogen
fixation, and how some forests can even manipulate it to speed up their
abilities to 'heal' themselves. Many tropical forests today have been exploited
for agriculture, mining, fossil fuel exploitation, and other human use.

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"It is therefore crucial to resolve whether nitrogen-fixing trees
can relieve nitrogen limitation and ensure fixation as an ecosystem service
during tropical forest secondary succession," writes Sarah Batterman of
Princeton University in a recent article published in the journal Nature.

A coalition of researchers from several institutions—Princeton
University, the Smithsonian Tropical Research Institute, Wageningen University
in the Netherlands, the University of Copenhagen in Denmark, Yale University,
and the German Center for Integrative Biodiversity Research—set out to
investigate the role played by nitrogen-fixing trees in tropical forest
recovery. They chose to study forest plots in Panama, under the auspices of the
Agua Salud Project that oversees the management of the watershed that supplies
the Panama Canal.

First, they estimated the biomass of the trees in a forest plot and
the proportion of each plant's biomass that was present in its roots, leaves or
stems. Using known root and stem (or wood) nitrogen levels, they calculated how
much of the tree's biomass consisted of nitrogen. In the case of leaves, which
are the most individually variable components of any plant, coauthor Dylan
Craven of Yale University assessed the average nitrogen levels present in the
leaves of over fifty species of tropical Panamanian plants.

Subsequently, the researchers also measured nitrogen fixation in
individual trees in each forest plot by counting the number of leguminous
nodules present on tree roots, which have a symbiotic relationship with
nitrogen-fixing bacteria. They then measured how quickly the bacteria in these
nodules could fix nitrogen.

Armed with these data, the scientists compared plots that had been
abandoned from pasture five, twelve, thirty, and eighty years ago, against
300-year-old mature forests.